Method of making a reverse osmosis element

ABSTRACT

A reverse osmosis water purification module has an enclosed cylindrical housing in which at least twelve purification elements are mounted in a uniform, spaced parallel fashion. Each element consists of a long flat member having a thermoplastic sheet backing formed into a slightly curved cross section and having a semi-permeable membrane disposed thereon. The backing has three-dimensional porosity for purified water such that water passing through the membrane portion of the element travels along the backing member to an end. The ends of the module are mounted in a collection manifold which is sealed off from the remainder of the housing interior. A method of manufacturing the elements continuously is also provided wherein a continuous length of flexible backing material is fed sequentially through a heater, a forming die to impart a curved cross-section to the backing material, an extruder wherein membrane forming fluid is extruded around the periphery of the backing, a chill bath and a heating bath. After passing through the various process steps, the continuous length of material is cut into elements of desired lengths.

This is a division of Ser. No. 074,570, filed 7/17/87, now U.S. Pat. No.4,844,805.

BACKGROUND OF THE INVENTION

This invention relates to elements mounted in a cartridge for separatingpure water from brine. More particularly, it relates to elements havinga reverse osmosis membrane disposed entirely around the surface of abacking member which is porous throughout its length, width, and depthbut which has sufficient structural strength to support the membrane. Inaddition, the invention relates to a method of manufacturing elementswhich comprises sequentially feeding a continuous strip of backingmaterial through a heater and cold iron to provide a curvedcross-section to the material, passing the material through an extrusionnozzle to extrude a reverse osmosis membrane on the backing material,passing the coated material sequentially into a chill bath and a heattreatment bath, cutting the material into discrete units and soaking theunits in glycerine.

The use of semi-permeable membranes to separate liquids from saltsdissolved therein is a well-known, commercially practiced technique. Inrecent years, the use of units in the home to purify water for drinking,ice making, and the like has become quite prevalent. Units used in thehome are generally relatively small, since they must be placedunderneath a sink or in other confined spaces, yet must maintainsufficiently high throughput to provide an adequate supply of purifiedwater. Accordingly it is necessary to design a unit which reasonablymaximizes the membrane surface area per unit of volume of the cartridge,since the volume of purified water produced by a membrane element isgenerally directly proportional to the surface area of the membrane incontact with the brine.

It is common for membrane elements to be mounted in parallel fashionwithin a cylindrical cartridge, with purified water being collected in aheader or manifold at one portion of the container, and with brinepassing through the container around exterior surfaces of the membranes.Cheng, U.S. Pat. Nos. 3,612,282 and Manjikian, 3,544,358, are examplesof such systems. Many other mounting configurations, such has spiralmounts of the type shown in Westmoreland, U.S. Pat. Nos. 3,367,504,Bray, 3,417,870, and Bairinja, 4,299,702, are also well-known.Difficulties have been encountered with spiral-wrapped membranes in thatseparating members between helical wraps impede the flow of brine andalso plug the membrane with salt. As a consequence, feed water must befiltered prior to feeding into a spiral-wrapped membrane. In addition,these membranes are relatively costly to manufacture and assemble. Also,in the past, in order to provide sufficiently dilute brine to themembrane surface, it has been necessary to have a continuous flushing ofbrine through the cartridge, resulting in a wasting of water. Continuousflushing of reverse osmosis units to drain has been criticized by manywater districts and is expected to be precluded by law in a number ofareas.

A substantial pressure drop is required across the reverse osmosismembrane, thus providing the driving force for separating the pure waterfrom dissolved salts. The membrane itself, however, is quite delicateand is required to be supported to withstand the pressure. The supportmust be sufficiently strong to resist deformation by the externalpressure, but must also be permeable to permit the flow of pure waterpassing through the membrane to be conducted into a collection chamberfor usage. Resistance to passage of purified water through the porousmembrane support will of course hinder the flow of water through themembrane, thus decreasing throughput of the element. Accordingly, muchresearch has been done to discover a optimum element design. Manycommercial units designed for purification of residential water havemembranes cast over tubular backings, with water being collected insidethe tube and being passed to a collection chamber.

In the present invention, a reverse osmosis element is made from abacking consisting of a stiff piece of porous plastic on which a reverseosmosis membrane has been cast. The element is porous laterally,longitudinally, as well as transversely; i.e., the backing is porous inthree dimensions and water may travel in any direction through thebacking once it has passed through the membrane. The membrane is castaround the entire periphery of the backing, and when it is mounted in acartridge, the water passes through the membrane and into the backing,and travels along the backing toward one end which is mounted in amanifold. The elements may be arranged in a plurality of differentgeometrical configurations within the cartridge; since the elements areflat and have a slightly curved cross-section, they are mounted inconfigurations similar to the vanes in a turbine, thus providingrelatively turbulent flow around both sides of the element and providinggood scouring of the blade surfaces. Because the elements themselves arerelatively rigid, they may be mounted in a cartridge without spacerswhich would touch the elements and possibly provide breaks therein whensalts accumulate.

Elements of the invention are manufactured by feeding a continuous bandof thermoplastic material and passing it through a heater to soften thematerial and then through an iron which sets the material to a desiredcurved cross-section. The band passes from the iron into an extrudingnozzle within which a reverse osmosis membrane is extruded onto thesurface of the backing, completely coating all peripheral areas. Thecoated backing is then passed through a chill bath to set the membrane,and a heating bath to heat treat the solid membrane. The band may thenbe cut into discrete elements and subsequently treated to remove watermolecules from the element, e.g., with a glycerine bath. The elementsare then mounted in parallel fashion in a cartridge in any of a varietyof manners, some of which are described herein.

Accordingly, it is an object of the present invention to provide areverse osmosis water purification system which is inexpensive tomanufacture, and which has a relatively low failure rate of membrane inservice. It is a further object of the invention to provide a reverseosmosis element consisting of an elongate flat member having a slightlycurved cross-section consisting of porous backing entirely coated with areverse osmosis membrane. It is yet a further object of the invention toprovide a reverse osmosis cartridge having a plurality of flat curvedelements mounted generally parallel therein, with one end of theelements extending through a header and into a collection chamberwherein purified water passing through the membrane backing iscollected. It is yet a further object of the invention to provide acontinuous method of manufacturing the discrete membrane elements havinga backing consisting of a flexible plastic band. These and other objectsart accomplished by the invention, several preferred embodiments ofwhich are described herein.

BRIEF SUMMARY OF THE INVENTION

A reverse osmosis element consists of an elongate flat plastic backingmember having three dimensional porosity having a reverse osmosismembrane cast around the entire periphery thereof. The element has acurved transverse cross-section. The element is mounted in a cartridgewith a number of other like elements mounted generally in parallelconfiguration. Water passes through the membrane to the porous backing,and travels along the backing to one end of the element which is mountedin or adjacent to a collection chamber. A method of manufacture is alsoprovided in which a continuous band of thermoplastic porous material ispassed through a heater, an iron, a membrane extruding nozzle, chill andheat baths, and a cutter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is best understood with reference to the drawings, inwhich:

FIG. 1 is a perspective view of an element of the invention;

FIG. 2 is a transverse cross-section thereof;

FIG. 3 is a transverse cross-section of another embodiment of theelement of the invention;

FIG. 4 is a transverse cross-section of an embodiment of the inventiondifferent from the one shown in FIGS. 1 and 3;

FIG. 5 is a descriptive diagram showing the process of manufacture ofelements of the invention;

FIG. 6 is a partial sectioned view showing the mounting of elements ofthe invention in a cartridge;

FIG. 7 shows a top-plan view of a header adapted for mounting elementsof the invention;

FIG. 8 is a partial side-section view showing mounting of elements ofthe invention in the header shown in FIG. 7;

FIGS. 9 and 10 show other mounting configurations of elements of theinvention within a cartridge; and

FIG. 11 is a side section view showing diagrammatically the mounting ofelements of the invention in a cartridge.

DESCRIPTION OF A PREFERRED EMBODIMENT

Referring first to FIGS. 1 and 2, reverse osmosis purification element 1is a generally flat, semi-rigid member 2 having a slightly curvedtransverse cross-section. The element has opposing side edges 4 and 6,and opposing ends 8 and 10. As shown in FIG. 2, element 1 consists of aninternal backing member 16 which is entirely coated around itsperipheral surface (except the ends thereof) with a reverse osmosismembrane. The membrane has an upper surface 12 and lower surface 14, andis slightly beaded as shown at 18 and 20 along the elongate edges 4 and6 to protect against any fuzz or sharp edges which might create faultsin the membrane, thereby permitting leakage of impure water into theinterior collecting portion of the element.

The term "slightly curved" is intended to include both uniform andirregular curvatures; the cross-section is preferably not flat but isarcuate in the manner of a decorative "mini-blind" or turbine blade asshown in FIGS. 1-4 and 7. The curvature provides structural strength,allows a higher area per unit volume of cartridge, and provides improvedflow patterns through the unit. The curvature is generally defined by anarc subtended by radii between about 10° and 60°, preferably betweenabout 20° and 45° of circumference. Each element may be made as long andwide as desired, but the thickness is very small relative to the otherdimensions. Typically, for the elements depicted in FIGS. 1-4, 7, and11, the thickness is less then about 5% of the width, and less than 0.5%than the length.

The backing member 16 may be any material which can have a sufficientamount of structural strength and which is porous to water in threedimensions; i.e., water traveling through the membrane can pass alongthe backing either longitudinally, laterally (across the width of themembrane), and transversely (through the thickness of the membrane).Because of the porosity feature, pure water passing through the membranereaches the backing and can travel along the backing to one end thereoffor collection. One method of collecting the water is shown in FIG. 8.

The term "semi-rigid" when applied to membranes of the invention isintended to describe an element having at least sufficient structuralrigidity and stiffness to stand upright without flexing without exteriorsupport except at the bottom for balance. While the backing materialsand membranes themselves are relatively flexible, stiffness is createdby forming the flat backing material into a slightly arcuate orcrescent-shaped cross-section, adding some structural strength to thebacking. Element stiffness is important so that when a plurality ofelements are mounted in close relationship inside a cartridge, theelements do not flex or sag, particularly under water flow, and touchother membranes, possibly creating areas of salt accumulation andmembrane damage. Accordingly, the membranes must be sufficiently stiffso as not to contact adjacent membranes when mounted in a cartridge. Ifdesired, the membranes may be made completely rigid or stiff, althoughthis is not necessary. Suitable membrane backing materials include"Simplex", manufactured by Hornwood Fabrics of Lylesville, N.C., whichis a melamine or epoxy-impregnated polyester material which is porous inthree dimensions. Another suitable material is a "Trecot"polyester-impregnated melamine also manufactured by Hornwood Fabrics.Other suitable backing materials are porous thermoplastics such aspolypropylene, UHMW and HD polyethylene, polyvinylidene fluoride,ethylene-vinylacetate, styrene-acrylonitrite, andpolytetrafluoroethylene. These materials are commercially availableunder the Porex trademark from Porex Technologies of Fairburn, Ga.Typical element dimensions include lengths of about 6"-36", preferably8"-20" and widths of 1"-2" depending on water production rate required.These elements can easily be mounted in parallel in conventionalcylindrical housing. Typically, the thickness of the backing is lessthan about 40 mils and ranges from about 10 mils for Trecot and 32 milsfor Porex. Thickness of the membrane coating usually runs from about 3mils to about 7 mils, and is preferably 5-6 mils, thus making themaximum desired element thickness of about 50 mils. Any semi-permeablemembrane film may be used to coat the backing according to theinvention; many reverse osmosis membrane materials are well known andcommercially available, including the cellulose acetate films disclosedin Loeb et al., U.S. Pat. Nos. 3,133,132, and Manjikian, 3,344,214. Themembranes are preferably cast onto the elements as a viscous liquid inthe manner described hereinafter.

FIG. 3 shows a different embodiment of an element of the invention fromthe one shown in FIG. 1, in that the backing material is doubled (i.e.,folded over) prior to application of the membrane. The backing, whichmay be a porous polyester such as Simplex or Trecot, is foldedlongitudinally into two layers 26 and 28. The membrane 24 coats theentire periphery of the backing. Purified water passing through themembrane can travel either along the interstices of the backing materialor along the cavity between the folded layers 26 and 28. A similarembodiment is shown in FIG. 4, in which element 30 is fabricated from afolded backing member having panels 32 and 34, which are mounted in a"sandwich" around a porous fabric member 36. Blotter 36 is a very porousmaterial which permits three dimensional flow of water with virtually noresistance at the flow rates contemplated by the invention. A membrane38 coats the entire peripheral surface of the element along its entirelength. One or more openings in the membrane at either or both endsthereof permits collection of the purified water.

Manufacture of elements of the invention is shown in the schematicdiagram of FIG. 5. The backing material for the element 40 is maintainedon a reel 42, thus providing a continuous supply of material. Thebacking material 40 is fed through a heating element 44 having a slot inblock 48 of the heater through which the backing material passes. Thesole purpose of the heater is to warm the backing material to atemperature of about 275°-325° F., preferably about 300° F. The heateris a simple resistance heating element and an air fan, similar to aportable hair dryer; any type of heater sufficient to warm the backingto this temperature can be used.

After exiting the heater 44, the backing material passes into a coldiron 50 which consists of a pair of opposing mated TEFLON coated jaws(shown as slot 52) which form the heated elongate backing material toprovide an arcuate transverse cross-section. In effect, the iron acts asa simple die to shape the band of material. The backing material ispulled through the heater and the die 50 by means of a drive unit 54which consists of a pair of rollers 56 and 58 mounted on shafts 60 and62. The rollers have a resilient surface which grasps the band ofbacking material, pulling it downwardly. Because the material has cooledsufficiently from passing through the iron, the drive unit does notdeform the cross-section of the backing material.

The backing material passes into a slot 66 in an extrusion nozzle 64within which viscous membrane-forming material is extruded onto thebacking material. The purpose of the extruding nozzle is simply to applyan even coat of membrane onto the backing; the particular design of theextrusion nozzle is not critical and can easily be designed by someoneskilled in the art. If desired, the nozzle can be shaped to extrudebeaded edges along the elongate edges of the band as shown in FIG. 2.

Coated backing 70 passes downward from extruder 64 into a chill bath 68which contains circulated water 72 at a temperature of from about 33° F.to about 37° F. The purpose of the chill bath is to form the membranefrom the membrane solution. The chill bath is preferably equipped with avent to remove Yolatile elements. The chill bath is equipped with arecirculating system including a pump, a thermostat-controlled coolingelement, and a filter to remove any particulate matter.

The coated band passes over guide rollers 74 and 76 continuously, andover a third guide roller 78 mounted above the bath, and into a heattreatment bath 80. The heat treatment bath contains water at atemperature of at least about 110° F., preferably from 120° F. to about180° F. for the purpose of shrinking the membrane and improving theselectivity of the membrane. The heat-treated band passes over a fourthguide roller 84, and exits the bath as treated element precursor 86. Theelement then passes to a cutter which separates transversely the coatedbacking material into elements of a desired length. The discreteelements are then treated in a glycerine bath (20% glycerine inde-ionized water) to displace trapped water in the element, permittingthe element to be stored at temperatures at which the water would freezeand damage the membrane. The bath is maintained at room temperature, andthe residence time of the element in the bath is at least about 1/2hour. Both the cooling and heating baths are equipped with a bleed drainto continuously remove a small portion of the bath to maintainconsistent low level of dissolved solids and contaminants from thecasting solution. An equivalent amount of non-contaminated watercontinuously replaces the fluid removed in the bleed system.

The travel rate of the band of backing material through the process ofthe invention is generally controlled by the extrusion element;extrusion is the slowest and most delicate part of the process. If theextrusion operates too rapidly, uniform coating may not occur, and if itoperates too slowly, the viscous treatment material may deform prior toentering the chill bath. Since the travel rate of the band is controlledby the extrusion step, the size of the chill bath and the heating bathis controlled to provide a residence time of the element of from about15 seconds to about 2 minutes, preferably about 1 minute, in the chillbath and in the heating bath from about 1 minute to about 5 minutes,preferably about 1.5 minutes.

FIG. 6 shows one method of mounting elements of the invention in acartridge. Elements 90 and 92, of the type shown in FIGS. 1 and 2, aremounted in a cylindrical tubular housing 94. At least 12, and preferablyat least 20 elements are mounted in the housing. A central mountingsupport tube 96 extends along the axis of the cylindrical housing, andhas a pair of mounting clips 98 and 100 having fingers which extend oneither side of the tubing to support the tube. In most installations,the mounting clips will not be needed as the stiffness of the elementsenable them to maintain a vertical position without additional support.In addition, the central support tube may be eliminated. The lower endsof each element abut a layer of fibrous material 106 which is porous andpermits the passage of water through the end of the element and into theconduit 112 of the purified water charge nipple 110. The fibrous pad orblotter 106 rests on the upper surface 104 of header 102, and is sealedfrom the interior chamber 109 of the housing by potting material 108.Elements are mounted in the header by aligning them in the desiredconfiguration with the ends of each element abutting the fibrous pad106, and pouring in a sealing means such as a liquid potting material(which may be an epoxy or other thermosetting resin material) whichprecludes impure water in the interior of the housing from contaminatingpurified water which passes through the conduit 112.

FIG. 7 shows a top view of a header into which elements of the inventionare mounted prior to enclosing within the housing of a reverse osmosispurification cartridge. The header 120 has a plurality of slots 122therein which can receive elements of the invention. Elements are placedin the slots as part of the manufacturing process of the cartridge. FIG.8 shows a partial side sectioned view of the header of the inventionshown in FIG. 7, with an element 130 being mounted in the slot 122. Thebottom wall of the header and the interior shelf 132 form a chamber 131for collection of purified water. The end of the element 130 extendsthrough the slot 122 and into the chamber so that water passing throughthe membrane of the element travels to the bottom portion of the elementand into the chamber 131, ultimately passing through conduit 128 forcollection. After all of the elements are in place in the header, thepotting material 138 is poured in to seal the interior of the housing ofthe cartridge from the collection chamber.

FIGS. 9 and 10 show other embodiments having different slot locations inthe bottom header into which the elements are mounted. In FIG. 9, agenerally concentric design is shown in which the header 142 has aplurality of generally concentric rings of elements 144, 146, 148, and150.

In FIG. 10, header 152 has several rows of elements in which the curvesurfaces are opposing. A row 154 of parallel elements extends across acentral portion of the cartridge. Interspersed between and offset fromrow 154 are opposing rows 156 and 158, dished upwardly in the drawing.Outer rows 160 and 162 have three parallel elements facing in a downwarddirection in the drawing. This arrangement provides for good scouring ofthe elements when feed water passes through the elements in turbulentflow, for instance, in a situation where the cartridge is mounted inline in cold water service in a residence, where operation of a faucetor a toilet would create substantial flow through the unit.

A schematic diagram showing mounting of the elements (in theconfiguration shown FIG. 7) in a cartridge is shown in FIG. 11.Cylindrical cartridge 151 has end cap closures 153 and 155. The waterinlet 159 is located in the side of the cartridge, permitting tangentialinflow of the water. Purified water is collected in a manifold aspreviously described and exits through nipple 157. Brine outlet 161 isshown in the top closure but may be located in the cartridgetangentially as is inlet 159 to provide for turbulent flow("turboflushing" action) of water through the cartridge. Thesecartridges are commonly mounted in a cold-water line in a residence suchthat when a valve is opened downstream (e.g., by opening a faucet orflushing a toilet), rapid flow through the cartridge takes place. Thevarious elements (e.g., 163 and 165) are mounted on a center tubesupport 167. When the unit is mounted under a sink, the cartridge ismounted with the purified water outlet at the top of the cartridge(i.e., upside down from the drawing in FIG. 11); this orientationpermits proper stratification of the feed water, which enters near thetop of the unit and proceeds downwardly as the salt concentrationincreases. In addition, any air that penetrates the membrane proceedsdirectly through the collection system and is removed as the system isused (i.e., as purified water is drawn from the collection tank).

Modifications and adaptations in the elements of the invention will bereadily apparent to those skilled in the art. Accordingly, the inventionshould not be considered limited by the foregoing description of apreferred embodiment thereof, but rather shall be defined only by thefollowing claims as construed to their fullest legal extent.

I claim:
 1. A method of manufacturing a reverse osmosis elementcomprising an elongate backing member having three dimensional porosityand coated with a semipermeable membrane comprising passing a continuouslength of backing material comprising a thin flexible strip ofthermoplastic material having three dimensional porosity through aheating element,forming the heated material to a desired curvedcross-section, continuously extruding a film of fluid semi-permeablemembrane forming material onto the backing material, passing coatedbacking material sequentially through a cooling bath and a heating bath,and cutting the continuous material into a plurality of elements.
 2. Themethod of claim 1 wherein the backing material is heated in the heatingelement to a temperature of from about 275° F. to about 325° F.
 3. Themethod of claim 1 wherein the backing material passes downwardly throughan extruding nozzle, and backing material having viscous membraneforming material extruded thereon is passed downwardly into a coolingbath containing a circulating fluid having a temperature from about 33°F. to about 37° F.
 4. The method of claim 1 wherein coated backingmaterial passes sequentially through a cooling bath having a temperaturefrom about 33° F. to about 37° F. and a heating bath having atemperature of at least about 110° F., with a residence time of fromabout 15 seconds to about 2 minutes in the cooling bath and from about 1minute to about 5 minutes in the heating bath.